This is the first application filed for the present invention.
Not Applicable.
The present invention relates to high-speed data communications systems and in particular to a method and apparatus for optimizing the performance of a data communications system.
It is well known that signals suffer degradation between the transmitter and receiver, due to various sampling and quantizing effects, and channel effects.
The sampling and quantizing effects comprise the distortion inherent in quantization of a received data signal, which may be a round-off or truncation error, errors introduced by saturation of the quantizer circuitry, and timing jitter. Generally, saturation may be avoided by using automatic gain control (AGC), which extends the operating range of the quantizer. Jitter includes any deviation of the position (i.e. the phase relationship) of the sampling clock with respect to the received data signal, and its effect is equivalent to a frequency modulation of the data signal. Timing jitter is generally controlled with very good power supply isolation and stable clock references.
Signal corruption introduced by the channel is due to such factors as noise, inter-symbol interference, dispersion, etc. The degradation of the recovered data signal quality with the channel induced errors is called xe2x80x9cthreshold effectxe2x80x9d.
If the channel noise is small, there will be no problem detecting the presence of a waveform, and the only errors present in reconstruction of the data signal will be due to sampling and quantizing noise. On the other hand, if the channel noise is large, the resultant detection errors cause reconstruction errors. Thermal noise interference from circuit switching transients can cause errors in detecting the signal pulses carrying the digitized symbols (data).
Intersymbol interference is due to the bandwidth of the channel. A band-limited channel tends to spread the pulses, and if the width of the pulse exceeds a symbol duration, overlap with neighboring pulses may occur.
Dispersion is the chromatic or wavelength dependence of a parameter, such as, for example, the distortion caused by different wavelengths of light within a pulse travelling at different speeds through a fiber. The pulse distortion in a fiber optic system may also be caused by some parts of the light pulses following longer paths (modes) than other parts.
The quality of a data signal is expressed in terms of a Bit Error Rate (BER) which is the ratio between the number of erroneous bits counted at a site of interest over the total number of bits received.
In the last decade, transmission rates of data signals have increased dramatically. For high rate transmission, such as at 10-40 Gb/s, signal corruption introduced by the transmission channel is a critical parameter. The demand for receivers with high sensitivity has increased progressively with the transmission rates. The receiver""s task is to determine which symbol was actually transmitted. For a given BER, the system performance is dependent upon the slicing level, defined also as threshold level, which is used to discriminate high and low levels of a received data signal. For example, a slicing level variation of only 8% can result in a variation of the receiver sensitivity of up to about 1 dB. Data signal recovery errors may develop as a result of an incorrect slicing level, or incorrect sampling clock/data signal timing (i.e. phase relationship) being selected.
Current optical receivers comprise an avalanche photodiode (APD), or a high performance PIN photodiode, typically coupled to a transimpedance amplifier. The transimpedance amplifier is a shunt feedback amplifier acting as a current-to-voltage transducer. The received signal is then amplified, and a data decoder (e.g. a single channel super-decoder) extracts a xe2x80x9ccleanxe2x80x9d data signal from the amplified received data signal. Generally, binary data decoders are provided with a fixed slicing level selected such as to provide the best error rate at a predetermined signal power level. However, a fixed slicing level cannot account for the effects of aging of the components, temperature variations, etc. As a result, higher power levels need to be transmitted to account for the above factors, which in turn diminish the length of the transmission channel.
As the requirement for essentially error free operation for fiber systems become more stringent, systems which allow bit detection errors to occur during a normal data signal recovery mode of operation are increasingly less acceptable. Driven by customer demand, sophisticated performance monitors are provided at the receiver site, which perform optimization routines for lowering the BER of the recovered data signal.
It is known to generate a control code at the transmission site which is then transmitted with the payload data over the communication link. Error detection is based, in general, on comparison between the transmitted and the received control code. Error correction is based on various algorithms which compensate for the specific error detected in the control code. This method is known as forward error correction (FEC).
A data decoder including a performance monitor is disclosed in U.S. Pat. No. 4,097,697 (Harman, issued on Jun. 27, 1978 and assigned to Northern Telecom Limited). This patent discloses a data decoder including a first differential amplifier which recovers the data signal by comparing the incoming signal with a fixed slicing level. A second differential amplifier compares the incoming signal with an offset slicing level to produce an errored signal. Both differential amplifiers are clocked by a recovered clock signal. The recovered data signal and the errored signal are compared to each other and the result is used to determine the degradation of the incoming signal.
U.S. Pat. No. 4,799,790 (Tsukamoto et al., issued Jan. 24, 1989 and assigned to Anritsu Corporation) discloses a device comprising a transmitter for launching signals of various wavelengths into a reference or test fiber, and a receiver. At the receiver, the phase difference between two adjacent wavelengths is measured for both the reference and test path for determining the delay of the respective wavelength.
None of the above patents is concerned, however, with providing a simple device and method for detecting and correcting errors in the recovered data signal using information in the data path itself. The receiver circuits described in the above patents rely on duplicate channels and pseudo-error detection.
The extent of signal degradations may be directly measured using an eye closure diagram, which is the graphic pattern produced on an oscilloscope when a baseband signal is applied to the vertical input of the oscilloscope and the symbol rate triggers the instrument time base. For a binary signal, such an eye diagram has a single eye which is open or closed to an extent determined by the signal degradation. For data recovery with low BER, an open pattern is desired. Changes in the eye opening size indicate intersymbol interference, amplitude irregularities, or timing problems.
U.S. Pat. No. 4,823,360 (Tremblay et al., issued Apr. 18, 1989 and assigned to Northern Telecom Limited) discloses a device for measuring chromatic dispersion of an optical fiber based on a baseband phase comparison method, using the eye closure diagram of the data signal received over the transmission link. The device described in this U.S. patent evaluates the transmission link performance using three slicing levels for recovering data. Two of the slicing levels are obtained by measuring on the eye diagram the level of xe2x80x9clong 0sxe2x80x9d and xe2x80x9clong 1sxe2x80x9d, respectively, for a preset error rate, and the third slicing level is provided in a selected relationship to the other two to produce recovered data signals.
The technique described in Tremblay et al. is based on generating xe2x80x9cpseudo-errorsxe2x80x9d on separate pseudo-error channels. The pseudo-errors give some idea of how error performance varies with the slicing level and, because they do not appear on the in-service transmission path, they do not affect service. Consequently, this technique can be used for dynamic control of in-service systems, and in fact has been used successfully in data transmission systems ranging in bit rate from OC-3 to OC-192 (0.155Gb/s to 10 Gb/s). Unfortunately, the separate pseudo-channels require additional high speed circuitry, and the pseudo-errors may not give a true reflection of error performance in the data path. Furthermore, at 40 Gb/s (OC-768) and above, the parallel decoders required to implement the method of Tremblay et al add excessive capacitance, and thus degrade the performance of the data decoder.
U.S. Pat. No. 5,896,391 (Solheim et al. issued Apr. 20, 1999 and assigned to Northern Telecom Limited) provides a method for recovering a data signal from an incoming signal received over a transmission network. The method provides for preparing a bit error rate (BER) map for a data decoder, determining, on the BER map, an optimal operation point for a provisioned BERprov value, and monitoring the data regenerator to function in the optimal operation point for providing a recovered data signal. In addition, the errors in the recovered data signal may be further corrected using current forward error correction circuitry.
The method of Solheim et al. offers one way of avoiding the use of parallel decoders, such as required by Tremblay et al. However, it entails the risk of perturbing the data decoder to an extent that creates an error burst of sufficient severity that the forward error correction circuitry is unable to correct the errors.
Accordingly, there remains a need for a method and apparatus capable of reliably optimizing the performance of very high speed optical transmission systems.
An object of the present invention is to provide a method and apparatus for optimizing the performance of a data communications system in which at least one parameter of the data communications system is dynamically adjusted.
A further object of the present invention is to provide a method and apparatus for optimizing the performance of a data communications system in which an error performance of the system is determined in the data path.
Another object of the present invention is to provide a method and apparatus for optimizing the performance of a data communications system in which an optimum level of a parameter is estimated using a block of sacrificial bits embedded within each data frame.
Accordingly, an aspect of the present invention provides a method of optimizing one or more parameters of a data communications system. The method comprises the steps of: receiving a data frame including a respective block of sacrificial bits having a known bit sequence; perturbing a value of a selected parameter of the communications system during reception of the block of sacrificial bits; calculating a bit error rate (BER) corresponding to the perturbed value of the selected parameter; and estimating an optimum value of the selected parameter based on the perturbed value of the selected parameter and the corresponding calculated bit error rate.
A further aspect of the present invention provides an apparatus for optimizing one or more parameters of a data communications system. The apparatus comprises: means for receiving a data signal of one or more data frames including a respective block of sacrificial bits having a known bit sequence; means for perturbing a value of a selected parameter of the communications system during reception of the block of sacrificial bits; means for calculating a bit error rate corresponding to the perturbed value of the selected parameter; and, means for estimating an optimum value of the selected parameter based on the perturbed value of the selected parameter and the corresponding calculated bit error rate.
Preferably, a value of the selected parameter is dynamically adjusted, in accordance with the estimated optimum value, to ensure reliable recovery of the data signal.
In preferred embodiments of the invention, the step of perturbing a value of the selected parameter comprises a step of applying a predetermined offset to a current value of the selected parameter.
The step of calculating a bit error rate corresponding to the perturbed value of the selected parameter preferably comprises the steps of: sampling at least a portion of the block of sacrificial bits using the perturbed value of the selected parameter. The sampled portion of the block of sacrificial bits is analyzed to calculate a corresponding bit error rate.
A preferred embodiment of the invention further comprises the steps of: receiving a plurality of data frames, each data frame including a respective block of sacrificial bits; calculating the bit error rate for each respective block of sacrificial bits using substantially the same perturbed value of the selected parameter; and calculating an average bit error rate in respect of the perturbed value of the selected parameter. In this case, the estimated optimum value of the selected parameter is preferably based on the calculated average bit error rate.
The perturbed parameter may be a level of a slicing level of a receiver unit of the data communications system. The perturbed value can be defined by a predetermined offset above a current value of the slicing level. Alternatively, the perturbed value can be defined by a predetermined offset below a current value of the slicing level.
The perturbed parameter may also be a timing of a sampling clock of a receiver unit of the data communications system. The perturbed value can be a predetermined phase advance of the sampling clock. Alternatively, the perturbed value can be a predetermined phase delay of the sampling clock.
The perturbed parameter may likewise be a transmission power of a transmitter unit of the data communications system. The perturbed value can be a predetermined power offset above a current value of the transmission power. Alternatively, the perturbed value can be defined by a predetermined power offset below a current value of the transmission power.
The step of perturbing a value of the selected parameter preferably comprises applying a selected one of first and second predetermined offset values to a current value of the selected parameter, to thereby define respective first and second perturbed values of the selected parameter. In these embodiments, the step of calculating a bit error rate preferably comprises the steps of calculating respective first and second bit error rates corresponding to the first and second perturbed values of the selected parameter. The estimated optimum value of the selected parameter is preferably based on the calculated first and second bit error rates.
Both of the first and second bit error rates may be calculated on a basis of a single block of sacrificial bits. In this case, the step of calculating respective first and second bit error rates comprises the steps of: sampling a first portion of the block of sacrificial bits using the first perturbed value of the selected parameter; analyzing the sampled first portion of the block of sacrificial bits to calculate the first bit error rate; sampling a second portion of the block of sacrificial bits using the second perturbed value of the selected parameter; and analyzing the sampled second portion of the block of sacrificial bits to calculate the second bit error rate.
The first bit error rate may alternatively be calculated in respect of a first set of one or more data frames, and the second bit error rate may be calculated in respect of a second set of one or more data frames. In this case, the step of calculating the first bit error rates comprises the steps of: sampling, using the first perturbed value of the selected parameter, at least a portion of each respective block of sacrificial bits of the first set of one or more data frames; analyzing the sampled portion of each respective block of sacrificial bits to calculate a corresponding bit error rate; and, calculating the first bit error rate by averaging the corresponding bit error rates calculated in respect of each data frame of the first set.
Similarly, the step of calculating the second bit error rate preferably comprises the steps of: sampling, using the second perturbed value of the selected parameter, at least a portion of each respective block of sacrificial bits of the second set of one or more data frames; analyzing the sampled portion of each respective block of sacrificial bits to calculate a corresponding bit error rate; and calculating the second bit error rate by averaging the corresponding bit error rates calculated in respect of each data frame of the second set.
Preferably, the first and second perturbed values are selected such that the corresponding first and second bit error rates are at least one order of magnitude (10xc3x97) higher than a bit error rate obtained using a nominal value of the parameter under normal operating conditions of the data communications network.
In embodiments wherein the parameter is the slicing level of a receiver unit of the data communications system, the first perturbed value is preferably defined by a predetermined first offset above a current value of the slicing level, and the second perturbed value is preferably defined by a predetermined second offset below the current value of the slicing level.
In embodiments wherein the parameter is a timing of a sampling clock of a receiver unit of the data communications system, the first perturbed value is preferably a predetermined phase advance of the sampling clock, and the second perturbed value is a predetermined phase delay of the sampling clock.
In embodiments wherein the parameter is a transmission power of a transmitter unit of the data communications system, the first perturbed value is preferably defined by a predetermined first power offset above a current value of the transmission power, and the second perturbed value is preferably defined by a predetermined second power offset below the current value of the transmission power.